The present invention relates generally to fault detection in operating electric motors and, more particularly, to a method for removing spurious current signatures in the process of motor current signature analysis of such motors.
Motor current signature analysis (MCSA) is a technique for monitoring and diagnosing mechanical problems associated with rotating machines driven by electric motors as well as fault conditions which may exist in the motors themselves. MCSA is based upon the fact that variations in motor geometry, load torque or speed will cause corresponding changes in motor current. The interpretation of the changes in motor current can be used to draw conclusions as to the health of the motor or load. Numerous papers and patents have been published which discuss the MCSA process. For example, Kliman et al., "Noninvasive Detection of Broken Rotor Bars in Operating Induction Motors," IEEE Transactions on Energy Conversion, Vol. 3, No. 4, December 1988, pages 873-879, discusses a non-invasive detection of broken rotor bars in operating induction motors. Another paper relating to the same subject is Kliman et al., "Methods of Motor Current Signature Analysis," Electric Machines and Power Systems, No. 20, 1992, pages 463-474. The latter paper lists a number of other references on the same subject. All of these papers discuss how MCSA can be utilized to identify fault conditions in either the motor or in loads connected to the motor.
In general, some systems utilizing multiple motors do not have a stiff voltage source that is capable of maintaining a constant voltage irrespective of variations in any one of the motors. As a result, motor current signature analysis may identify a problem with a particular motor when in fact the problem arises from another motor in the same system. Consequently, a motor can be detected as having a broken bar or other defect, be taken off line, and then, on physical examination, be found to not have a defect. Signals representative of such false defect detections unpredictably appear and disappear and are sometimes referred to as "ghosts."